This invention relates in general to the transfer of electrical power between two separated locations by means of transmitting and receiving electromagnetic waves, and more particularly to an antenna array for receiving dual polarized electromagnetic waves with high efficiency over a wide range of angles of incidence.
Research in the area of remotely powered mobile systems has centered around the requirement for cost effective means to receive and convert transmitted electromagnetic power into direct current power when the transmitter and receiver are moving relative to one another.
For example, it has been proposed that an electromagnetic power reception and conversion system could be implemented for transmitting propulsive and communications payload power in the 2.4-2.5 GHz microwave ISM band to a lightweight electrically-powered aircraft circling over a fixed ground antenna system for continuous periods of weeks or even months at a time.
One prior art electromagnetic power reception and conversion system is described in U.S. Pat. No. 3,434,678, and is referred to as a linear rectenna. This device consists of an array of linearly polarized half-wavelength dipole antennae, each followed by a conversion system consisting of wave filters and rectifier circuits.
In order to achieve high efficiency power collection with the linearly polarized rectenna described in the prior art patent, the transmitted electromagnetic field must itself be linearly polarized. In addition, the polarization orientation of this field must be maintained parallel to that of the rectenna dipoles, or vice versa, in response to changes in the orientation of the receiving rectenna relative to the power transmission antenna, or due to Faraday rotation of a polarized beam transmitted through the ionosphere, etc. In other words, expensive and complex polarization tracking equipment must be provided at either the transmitting antenna or the receiving rectenna in order for the system to operate properly with high efficiency power collection.
An improvement in linearly polarized rectennae is described in an article entitled "Design Definition of a Microwave Power Reception and Conversion System for Use on a High Altitude Powered Platform" NASA/CR/156866, by W.C. Brown, published in 1981. According to the Brown article, a linearly polarized rectenna is disclosed in the form of a thin-film printed circuit. This type of linear rectenna has many desirable characteristics which were not possessed by earlier prior art rectennae constructed of discrete components; such as those described in a further article of W.C. Brown entitled "The History Of The Development Of The Rectenna", publication of the S.P.S. Microwave Systems Workshop, Rectenna Session, Lyndon B. Johnson Space Center, Houston, Texas, Jan. 15-18, 1980.
With the exception of the inclusion of rectifier diodes, all components of the improved rectenna were etched on a single thin-film dielectric sheet. Therefore, when compared with earlier discrete component rectennae, the potential fabrication costs for volume production are very low. Furthermore, the structural weight of the thin-film rectenna is very low (eg. typically less than 100 grams-per-square-meter), and the thin-film fabrication is very flexible and can be conformed to curved surfaces such as aircraft wings, etc.
However, the improved rectenna disclosed by W.C. Brown also suffers from the principle disadvantage of well known prior art discrete component rectennae, which is that for high efficiency power reception the polarization orientation of the field should be parallel to that of the rectenna dipoles, resulting in the necessity of expensive and complex polarization tracking equipment.
A further prior art system is described in U.S. Pat. No. 3,681,769 which teaches the use of multiple phased arrays of orthogonal dipoles disposed on separate planes and interconnected via transmission lines for transmission of radio signals in a dual polarized beam.
However, this prior art approach suffers from poor efficiency performance due to shielding effects caused by the transmission lines. As an example, when two thin-film rectennae of the type and dimensions described in the latter mentioned article by Brown are laid out in two orthogonal foreplanes as taught by U.S. Pat. No. 3,681,769, it can be readily shown that approximately 30-40% of the power in one polarization is prevented from being received by the transmission lines of the other foreplane. Furthermore, such phased arrays of orthogonally disposed dipoles are subject to extremely poor directivity when applied to systems in which the angle of beam incidence varies (e.g. in systems characterized by relative movement between the transmitting and receiving stations, such as in an electrically propelled airborne transportation system). This is because the directivity of such arrays is proportional to the ratio of the wavelength to the dimensions of the array.
In addition, as the separation between the planes is reduced to electrically small values, as would often be necessary for conformal applications, mutual coupling between the dipoles and transmission lines is known to occur, thereby reducing the reception and conversion efficiencies even further.
One approach to eliminating the prior art requirement for polarization tracking equipment has been to replace the linearly polarized dipole array rectenna with a circularly polarized microstrip antenna array, as described in U.S. Pat. No. 4,079,268. However, it is believed that such a proposed microstrip antenna array would be incapable of achieving the 85% or better reception efficiencies which are characteristic of linearly polarized thin-film dipole rectennae.